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I am wanting to learn (memorise) some of the tree of life; the classification of organisms. So I have come across a lot of kingdoms in my research. Basically, I don't want to waste my time learning the wrong thing, so my question is this. Is there an official or most recent system of classification, some authority in organism classification that we look to? After kingdoms it seems pretty clear, the phyla in (say Animalia) seem pretty defined and uncontraversial.
These are all of the 'kingdoms' I have found while researching. Some are called kingdoms in one source and not another. I know that some overlap others. This is just to throw every possible example out there.
Animalia, Apusozoa, Archae, Archaebacteria, Archezoa, Bacteria, Biliphyta, Chromista, Chromophyta, Ciliofungi, Cryptophyta, Eufungi, Euglenozoa, Excavata, Filastera, Fungi, Monera, Nucleariida, Plantae, Proteoarchaeota, Protista, Protozoa, Rhizaria, True bacteria, Viridiplantae
I don't ask that you go through them and explain them. Only what the actual system of classificating organisms is. On that note. As far as I'm aware, there's two types of domain systems. Two are Eukaryota, Archae/-bacteria, Bacteria. Another is Eukaryota, prokaryota. A clarification on this too would be appreciated.
2.3: Diversity of Life
- Contributed by Suzanne Wakim & Mandeep Grewal
- Professors (Cell Molecular Biology & Plant Science) at Butte College
The collage below shows six kingdoms into which all of Earth's living things are commonly classified. How many species are there in each kingdom? In a word, millions. A total of almost 2 million living species have already been identified, and new species are being discovered all the time. Scientists estimate that there may be as many as 30 million different species alive on Earth today! Clearly, there is a tremendous variety of life on Earth.
Figure (PageIndex<1>): Six kingdoms of life: Archaea, Bacteria, Protista, Fungi, Animalia, and Plantae
Six Kingdoms of Life
By Beth Rowen
Every living creature on Earth belongs to a kingdom. Scientists debate how many kingdoms there are, with many arguing that there are six. Here is how the six kingdoms are organized. Or, seehow some other scientists categorize life into five kingdoms.
Eubacteria are single-celled organisms that don?t have a nucleus. Bacteria make up the entire kingdom. There are more forms of bacteria than any other organism on Earth. Some bacteria are beneficial to us, such as the ones found in yogurt. Others can cause us to get sick.
Protists are mostly single-celled organisms that have a nucleus. They usually live in water. Some protists move around, while others stay in one place. Examples of protists include some algae, paramecium, and amoeba.
Fungi are usually motionless organisms that absorb nutrients for survival. They include mushrooms, molds, and yeasts.
Plants contain chlorophyll, a green pigment necessary for photosynthesis, a process in which plants convert energy from sunlight into food. Their cell walls are made sturdy by a material called cellulose, and they are fixed in one place. Plants are divided into two groups: flower- and fruit-producing plants and those that don?t produce flowers or fruits. They include garden flowers, agricultural crops, grasses, shrubs, ferns, mosses, and conifers.
Animals are the most complex organisms on Earth. Animals are multi-celled organisms, eat food for survival, and have nervous systems. They are divided into vertebrates and invertebrates and include mammals, amphibians, reptiles, birds and fish.
The Plant or Plantae kingdom encompasses all flowering plants, mosses and ferns. Plants are multi-celled, complex organisms and are considered Autotrophic. This means that plants create their own food through photosynthesis. The Plant kingdom is thought to be the second largest with over 25,000 known species.
The largest kingdom of organisms is the Animal or Animalia kingdom. This kingdom is made up of complex, multi-celled organisms ranging from sea sponge colonies to elephants. All organisms in the Animal kingdom are Heterotrophs meaning, unlike plants which produce their own food, animals feed upon other organisms. The Animal kingdom is the world's largest with over one million known species.
History of Kingdom in Taxonomy
Carl Linnaeus first used the term in the 1700’s to describe the highest orders of life. The two main groups of life, as he saw it, were the kingdom Plantae and the kingdom Animalia. Modern microbiology and genetics have revealed a much different picture. By the 1990’s it was generally accepted that all of life should first be broken into three domains. These domains, Bacteria, Archaea and Eukaryota, represent basic differences in cell structure, formation, and survival. As seen below, each major kingdom can fit within the domains, all of which are thought to have a common ancestor.
Some of the taxon represented here are actually phylum, and don’t represent an entire kingdom. Advances in cladistics and genetics revealed some individual phylum to be closer to a kingdom. As discussed below, this lead to changes in the way we describe and view a kingdom. Over time, many different versions have evolved from Linnaeus’ first taxonomy. Scientists in the 1800s finally recognized protists and other single celled organisms in their own kingdom, and separated the fungi kingdom from the plant kingdom. More recently, genetic evidence has revealed the bacteria and protists to contain a huge variety of genetics and adaptations.
This spurred the 3 domain system, which is typically accepted today. However, studies are continually being done on the legitimacy of the kingdoms and their organization. Some of this stems from new developments and studies which add to the picture. This constant addition of information is continually reforming taxonomy, and as such the definition and scope of ‘kingdom’ is constantly changing.
Taxonomy is the practice of identifying different organisms, classifying them into categories, and naming them. All organisms, both living and extinct, are classified into distinct groups with other similar organisms and given a scientific name.
The classification of organisms has various hierarchical categories. Categories gradually shift from being very broad and including many different organisms to very specific and identifying single species.
There are eight distinct taxonomic categories. These are: Domain, Kingdom, Phylum, Class, Order, Family, Genus, and Species.
With each step down in classification, organisms are split into more and more specific groups.
For example, all of the animals in the Kingdom Animalia are split into multiple phyla (plural of phylum). All of the animals in the phylum Chordata are split into multiple classes such as mammals, reptiles, and amphibians.
The broadest category splits all organisms into three groups called ‘Domains’. The three Domains of life are Bacteria, Archaea and Eukaryota.
For a long time, all life was separated into five or six kingdoms. These included kingdoms such as animals, plants, fungi, protists, archaea, and bacteria.
With new genetic data, we now know that some protists are more closely related to animals, plants, and fungi than they are to other protists. This suggests that the protist kingdom could be separated into multiple kingdoms. Thoughts are similar for the bacteria and archaea kingdoms.
A phylum (plural phyla) is still a very broad classification but it splits kingdoms into multiple groups. An example of phyla from the animal kingdom is Arthropoda which includes all insects, spiders, crustaceans, and more. All vertebrate animals belong to one phylum called ‘Chordata’. Invertebrates are separated into many different phyla.
A class is the next level down. As mentioned earlier some classes from the phylum Chordata include mammals, reptiles, and amphibians. Arthropod classes include the likes of insects and arachnids (spiders, mites, and scorpions).
Order and family
From class, organisms are placed into an Order and then a Family. Using grasses as an example from the plant kingdom, they belong to the order Poales and the family Poaceae.
Genus and species
The final two categories are genus and species. The genus and species that an organism belongs to are how an organism receives its scientific name. This naming system is called ‘binomial nomenclature’ and was invented by a brilliant biologist named Carl Linnaeus.
An identified species is placed into a specific group in each of these categories. For example, the taxonomic classification of humans is:
Species: Homo sapiens
To remember the order of the taxonomic hierarchy from domain to species, people often use mnemonics to make it easier. The phrase that I was taught and still use to help me remember is ‘King Phillip Came Over From Germany Swimming’. There are many different phrases people have come up with. If you’re not keen on the sentence I use and want another one, take a look at these taxonomy mnemonics .
Carl Linnaeus was a Swedish naturalist from the 18th century and is considered the father of taxonomy. It was Linnaeus who first began to separate organisms into hierarchical categories. He also developed the system that we use to name new species called ‘binomial nomenclature’. Linnaeus is credited with identifying over 10,000 different plant and animal species in his lifetime, more than any other biologist.
When Linnaeus developed his system of hierarchical categories, he called it ‘Systema Naturae’. It contained three kingdoms, classes, orders, genera, and species. We have since added two more categories – domains and phyla.
Linnaeus’s original classification had three kingdoms – animals, plants, and minerals (natural, non-living elements). We now only use this system for classifying organisms and we have since separated all of life into more than two kingdoms.
Binomial nomenclature is the method that we use to uniquely name every different organism on Earth, living or extinct. All organisms have a scientific name that includes two Latin words.
The two words are made from the names of the genus the species belongs to and a second word to separate each of the species within the same genus. The second word is known as the ‘specific epithet’. Hence, the scientific names of all organisms are made from the name of their genus and a specific epithet.
For example, the scientific name given to humans includes their genus Homo and the specific epithet sapiens. The overall name is Homo sapiens .
Scientific names are also written in either italics or underlined.
Taxonomy is not a perfect science and, as you will find out, there is a lot of disagreement and uncertainty about the structure of taxonomic classifications. In general, however, taxonomy is a great way to quickly learn about how an organism slots into the tree of life.
Last edited: 30 August 2020
Want to learn more?
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This Archaea domain contains single-celled organisms. Archaea have genes that are similar to both bacteria and eukaryotes. Because they are very similar to bacteria in appearance, they were originally mistaken for bacteria.
Like bacteria, archaea are prokaryotic organisms and do not have a membrane-bound nucleus. They also lack internal cell organelles and many are about the same size as and similar in shape to bacteria. Archaea reproduce by binary fission, have one circular chromosome, and use flagella to move around in their environment as do bacteria.
Archaea differ from bacteria in cell wall composition and differ from both bacteria and eukaryotes in membrane composition and rRNA type. These differences are substantial enough to warrant that archaea have a separate domain.
Archaea are extreme organisms that live under some of the most extreme environmental conditions. This includes within hydrothermal vents, acidic springs, and under Arctic ice. Archaea are divided into three main phyla: Crenarchaeota, Euryarchaeota, and Korarchaeota.
- Crenarchaeota include many organisms that are hyperthermophiles and thermoacidophiles. These archaea thrive in environments with great temperature extremes (hyperthermophiles) and in extremely hot and acidic environments (thermoacidophiles.)
- Archaea known as methanogens are of the Euryarchaeota phylum. They produce methane as a byproduct of metabolism and require an oxygen-free environment.
- Little is known about Korarchaeota archaea as few species have been found living in places such as hot springs, hydrothermal vents, and obsidian pools.
What are kingdoms?
To help us understand the number of living things on the planet, scientists organise them into groups, like 'the animals' or 'the plants'. This is called classification.
To start with, all living things are divided into large groups called 'kingdoms'. Scientists haven't quite agreed how many kingdoms there are, but many think that there are five: the monera, the protoctista, the plants, the fungi and the animals.
You probably know about the plants, animals and fungi (like mushrooms and yeast), but the monera and the protoctista may be new to you.
Click on the kingdoms below to find out more about each group.
Now that you've got the idea of kingdoms, you can now go on to find out more about further levels of classification. The rest of Animal I.D. explores classification of the animal kingdom.
Can you answer these questions? Click on a question to discover the answer.
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The Three Domains
The Archaea are a domain of bacteria-like organisms, but they have a distinct biochemistry which distinguishes them from bacteria. While they still have bacteria-like DNA and no organelles, they also have several differences which put them into a completely different domain. Their cell walls, and the RNA they produce, are significantly different from that found in the domain Bacteria.
The organisms found in the domain Archaea are often extremophiles. These organisms thrive in environments which other organisms find hostile. This could be a highly salty environment, one of high or low temperature, or even one with chemicals that are toxic to other organisms. While organisms in the domain Archaea were often considered distantly related to bacteria because of their weird tendencies, other scientists have hypothesized that they are more closely related to the organisms in the domain Eukarya.
While there are millions, if not billions of species of bacteria, there are a handful that are significant to humans. For instance, many of the organisms living in your gut helping you digest you food come from this domain. We need many bacteria for the vitamins and nutrients they produce. However there are several species, such as the bacteria which cause cholera or meningitis, which are highly detrimental to human health. Even the plague, which has killed off massive amounts of the human population throughout history, was caused by organisms in this domain.
The domain Eukarya represents, well, everything else. The organisms in this domain are eukaryotic, meaning they have a membrane bound nucleus and organelles. In separating different tasks of life into these membrane-bound chambers, eukaryotes are able to increase their efficiency and host a different set of chemical reactions. For instance, mitochondria are energy-transferring organelles which enable organisms in this domain to undergo aerobic respiration. This allows them to process glucose, a necessary sugar for life, much more efficiently. As such, the organisms in the domain Eukarya have expanded to a large number of forms.
The eukaryotes include everything with organelles and a nucleus. This means that the microscopic amoeba and the largest animal on the planet, the blue whale, are both in this domain. Plants and fungi also share this domain. This may give a scale for how variable a domain can be. While it is easy to demonstrate within this domain because the differences are largely physiological, each domain has this much variability within it. The other domains, Bacteria and Archaea, contain largely biochemical variability, as their physiological variability is limited by being unicellular.
Phylum (plural: phyla) is a taxon used in the scientific classification of life. "Phyla" is adopted from the Greek phylai, the clan-based voting groups in Greek city-states. Phyla represent the largest generally accepted groupings of animals and other living things with certain evolutionary traits, although the phyla themselves may sometimes be grouped into superphyla (e.g. Ecdysozoa with eight phyla, including arthropods and roundworms and Deuterostomia with the echinoderms, chordates, hemichordates and arrow worms). Informally, phyla can be thought of as grouping animals based on general body plan Ώ] , e.g., having bones vs. having an exoskeleton. This is morphological grouping. But despite the seemingly different external appearances of organisms, they are classified into phyla based on their internal organizations ΐ] . For example, though seemingly divergent, spiders and crabs both belong to Arthropoda, whereas earthworms and tapeworms, similar in shape, are from Annelida and Platyhelminthes, respectively. Although the International Code of Botanical Nomenclature allows the use of the term "Phylum" in reference to plants, the term "Division" is almost always used by botanists.
The best known animal phyla are the Mollusca, Porifera, Cnidaria, Platyhelminthes, Nematoda, Annelida, Arthropoda, Echinodermata, and Chordata, the phylum humans belong to. Although there are approximately 35 phyla, these nine include the majority of the species. Many phyla are exclusively marine, and only one phylum is entirely absent from the world's oceans: the Onychophora or velvet worms. The most recently discovered phylum is Cycliophora Α] found in 1993 only three phyla were discovered in the last century.
The Cambrian explosion was a great flowering of life forms that occurred between roughly 542 and 530 million years ago during this time all modern phyla (and many now-extinct ones) were established. Β] Over time the roles among different phyla have varied. For instance, during the Cambrian, the dominant megafauna, or large animals, were arthropods whereas now they are vertebrates (chordata). Γ]
Scientific classification describes the following levels of organization (taxons) for classifying life forms: Kingdom, Phylum, Class, Order, Family, Genus, Species, Subspecies. Thus Phylum is a rather broad classification. Also, the Three Domain System is now widely used and adds the level of Domain above Kingdom as the broadest classification.